Carpal Tunnel

The carpal tunnel is a narrow, rigid passageway located on the anterior side of the wrist. It serves as a conduit for the median nerve and nine flexor tendons that pass from the forearm into the hand. Surrounded by carpal bones and the overlying flexor retinaculum, the tunnel plays a critical role in hand function but is also a common site for nerve compression.

Location

The carpal tunnel is situated on the palmar side of the wrist. It extends from the distal end of the forearm to the proximal palm and lies deep to the skin and palmar fascia.

Structure

The carpal tunnel has distinct anatomical boundaries and contents:

• Roof: Flexor retinaculum (transverse carpal ligament) — a strong fibrous band stretching from the pisiform and hook of hamate (ulnar side) to the scaphoid and trapezium (radial side).
• Floor and Walls: Concave arch of the carpal bones — including the scaphoid, trapezium, lunate, triquetrum, pisiform, and hamate.
• Contents:
  • Median nerve
  • Four tendons of flexor digitorum superficialis
  • Four tendons of flexor digitorum profundus
  • Tendon of flexor pollicis longus

Function

The carpal tunnel allows the safe and efficient passage of nerves and tendons from the forearm into the hand. It:

• Protects the median nerve and flexor tendons during wrist movement.
• Maintains the alignment of flexor tendons for effective finger and thumb flexion.
• Supports efficient force transmission during grip and manipulation tasks.

Innervation

The median nerve passes through the carpal tunnel and supplies motor innervation to the thenar muscles and lateral two lumbricals, and sensory innervation to the palmar side of the thumb, index, middle, and lateral half of the ring finger.

Blood Supply

• Supplied by small branches of the radial artery, ulnar artery, and anterior interosseous artery.

Relations

Superficial to the tunnel lies the palmaris longus tendon (if present), ulnar artery, and ulnar nerve (which do not pass through the tunnel).
Deep to the tunnel are the carpal bones forming the concave floor.

Surface Anatomy

The carpal tunnel corresponds to the depression at the base of the palm near the wrist crease. Clinically, it is approximated between the thenar and hypothenar eminences.

Development

The carpal tunnel forms as the wrist bones and soft tissue structures develop from the limb bud during embryogenesis. The flexor retinaculum forms over the deep carpal arch, converting it into a tunnel.

Anatomical Variations

• Accessory muscles (e.g., palmaris profundus, accessory flexor tendons) may occasionally pass through the tunnel and predispose to compression.
• Variation in tendon arrangement or bifid median nerve may be present in some individuals.

Clinical Significance

• Carpal Tunnel Syndrome (CTS): Compression of the median nerve within the tunnel, resulting in pain, tingling, numbness, and weakness in the hand, particularly the lateral 3½ digits.
• Tinel’s Sign: Percussion over the carpal tunnel causes tingling in the median nerve distribution.
• Phalen’s Maneuver: Wrist flexion exacerbates symptoms, helping diagnose CTS.
• Treatment: Includes wrist splinting, corticosteroid injections, or surgical decompression (carpal tunnel release) by cutting the flexor retinaculum.

Comparative Anatomy

While similar flexor tunnels exist in quadrupeds, the human carpal tunnel is more refined due to enhanced thumb use and hand dexterity. The development of a closed tunnel structure aids in the precision and strength required for tool use and fine motor tasks.